Method of making optical transmission filters by thermal evaporation



Dec. 14, 1948. AXLER ET AL 2,456,241

METHOD OF MAKING OPTICAL TRANSMISSION FILTERS BY THERMAL EVAPORATION Filed Nov. 22, 1946 2 Sheets-Sheet. l

INVENTORASV Meyer fl vizfr Ken/rail: Z FW Dec. 14, 1948. AXLER ET AL 2,456,241

METHOD OF MAKING OPTICAL TRANsMIssION FILTERS BY THERMAL EVAPORATION Filed Nov. 22, 1946 2 Sheet-Sheet 2 INVENT S May/er hf Ar r KM!!! 17' Y *MJMLMMM ATTORNEYS Patented Dec. 14, 1948 i ETD" PATENT OFFICE G IW E snmmsmx THERMAU l lvuimn;A: tr angl -carom, anau'gennemFrrii-ip ruckalinegn. 'Y., as ignors w F rearm mm 0651x162, aicdriidi'aifibn of NwYisik ApplieatiUn 'NoVember ZZ, IMWZSMQFNW (11 ,562 i 8 Claims. c1.i1"'17i) This invention relates to an imprcrv'ed opticgl line'sui fece orr-wliiclithe next layer maybe stic filter -operating bylightwrayinterifereneeiag dab cess f iil-ly' deposit ed.- a sorption and.. toa, method--and fer the One -cittkgyprincipal dimcu-ltieskii-therto exp production thereof v ll enceq im the manufelcture m; interference filters:

optical interference I filters oi alternate layers; at acteristics qt the filte1 s-= with ageand" especially metallic an nonmetall-iq materials} jo'rf example follbwil' lgw the agppl ica ibn theret o of mechanical.

as descr-ibed in Germa tbliatent} N9 Q 7 16,153, pub-- pgetection ill- $1 18 term 0 cemented cover plate. lished -Ja nuary' mg. 194 2,-but=ithasbeenioufld The usual" intepierence filter comprises aJgl'etSs most difficultinpractice tbprod uce'met allic and 19 support upen which a e successively deposited 5,; nonmetallie layeiqs ofthereguired -homcgeneity,. first semi-transparent layernf silver or almriii unifcnmity and-integrity. Aec ordingly; .it is tlie- 1mm; en intermediate layer (if: magltlesiumflflllQ- principal object cf the; p 'esnt invention to pro ride,:-ce;lc :=iu m flu o ide U or sirhilai: dielectric mate vide an improvedmethed and -meails-fdec0n9mi 'ri alr-am l-a' -third layer-of silver pr aluminum: In

caflly-'manuiectgring;lnterfgencef ifilters of; the order-thattlieifiltermay be sufiicientlyrug dtc typegenerally described-inthe 'said; qer m patwithstand ndr nal pse;,-

entso= that such filters may be produeed in rea the microseopic'elly thin layers by mean's of agla ss sonable qua r tity with hi'gli quelityand witli' pre cover plate. Ttgecqver plate i 'sutlly takes the determined transiriliss'icm chai acteristics' form ofa blocki ng filter which, lay: absorptien;

is necessary to protect B'ecause thef filter leyers lie-bf extreme 20 serves-t2)ci t' ciftrainsmission peaks:ofundesired thinness" andbecaus'e th'e'sii s1ve1ayers"aire '0'forders of interference. cover plate is sematerials having widely" diff nt"pli'$ sic;l ejrties;iti'lias beriqiificfil cured yv-itli an. optical cement'to the uppermost; metaliic' layer; Application of the cement andether'dntd: a; permafierwand"uursliie structurz ngissigp in-the a u of a xm umtga smission of In accordance witntjhef invention tniayers m av the ter, and; in the l alffwidth of the transmis befonvenien'tlysecliiedbfsudceiy"evaiibrftibfi sion.-pea k; Commonly-the Values of maximum.

increase,constitiitirigfqetriorition in' thetqiialit'y; oi-tlie fllters. In additiqr ,-.alny siich changeset tcfe're produced filters indicates that" tli'e' diifl'ciiI- I conditions" of deposition according to the nature ti sare diig-tc defects in the structiiie-O'f tli'e' oft-tien fiiltfitl beifig:depibslteci sb tfiat ezcll-Taster metel lic and di'electric' layers, especially of the" will pdss'ess ana retain" aumlmrm tmcknessssnd' intermediate layer of dielectric materialand' of me as the nex t'lety'er toti: deposited. nai'ce'r'de 40 laid The dilebtfidlyer; when-deposited" posited layers cannot havei'g-oow characteristics on a cool s rfce cfei'ther glass or metal, {06in} unless the layers onvwh-ichthe$i ar"depo'sited" mainly exhibits; grfiera; sliort a'girig'; ai'rietwcrkqff providewavgood foundation:and'supportr weeks-averaging frb' 10 td-IOO microns in' length;

-We havefiiscovered that the critieal fitfizor Dielectric ldyers'sd depcsitd 'o'n gllsscr metallic hp d l y s df'm tefialsofisuch wide surl'ce'smt roo'rn temperature c mmon1ymve a;

1y. differing physical properties as metals and' nonflaky;- polycrystalline structure of lbw r'sistgific metals-is the temperaturedf the surtace on whichtc -abrasicn and 'pgorly bgnddQtotl'ielbwei' layer the evaporated riiateriial cdhdensemandthatme or supporti-ng surfacet Such dielectric layi's'ar;

tal li c layrs" are best depUsi-ted-on cold surfaces moreovemstrongly' hygroscopic, so that 'tl'iefiriwhile dielectric layefs are best deto'sited on hot isnea fiitepempodyin l such; a dielectric 1ayer,l;de'-

surfaces By cbfitrolling the temperatures--0f the: spite precautions to seal the -ltyer's' against" the layi' nottdnlyacquires the desi 'ed chaii acteflstics develqpbgrbss" or of unifo'rin thickfiessralnd=ireedommm craeks or lefigthy I t H flakiness, but also wesems al smootmnoncrystah I The-imperfect mechanical structure of dieleb acksup to a qulrte'r ai an inch in 3 tric layers so deposited at room temperature is particularly prejudicial to the production of uniform interference filters of high quality because of its effect on the overlying metallic layer. With a flaky, cracked dielectric layer such as results from the evaporation of the dielectricmaterial onto the first metallic layer while the latter is at room temperature, the outer metallic layer will conjunction with the accompanying drawings in likewise be lacking in compactness and uniform' ity. The cracks in the dielectric layer can readily r be of such dimensions that the metalcondensingthereon during the evaporation of the outer metallic layer will fail to bridge these cracks. The

result is a filter showing marked scattering in a strong beam of light, reduced transmission and an opalescent surface even to the naked eye." When the protective cover is then cemented onto a filter so produced, serious changes in the maxi.-

which i Fig. l-shows a perspective view of an apparatus adapted to the practice of the invention;

Fig. 2 is a partial elevation of the filter carrying fixture shown in Fig. 1;

' Fig. 3' showsa highly magnified view of the ap- 'pearance of a filter produced according to prior methods and exhibiting the defects which are overcome by the present invention;

mum value of transmission and in the half width of the transmission peaks occur. These are believed to be due to a migration of the cement through the upper metallic layer whereby the thickness of the dielectric layer is eifectively increased and the density of the upper metallic layer changed.

In order to produce a dielectric layer of superior mechanical and hence of superior optical properties, it is necessary to proceed with the] deposition thereof upon a heated surface. 'We have discovered, however, that the entire process of filter manufacture should not be carried out with the successive layers or'surfaces at an elevated temperature. The deposition of the first metallic layer on a heated surface results'not in a smooth and homogeneous metallic layer, but in a metallic layer with a seeded surface which itself" promotes the appearance of cracks in the dielectrio layer to be laid down thereon.

We have discovered that if the first metallic layer is deposited upon the glass or other trans: parent support Whilethe latter is at room temperature and the support and initial layer are thereafter heated .so that the, dielectric layer is deposited while the support and first layer are at an elevated temperature, the difiiculties are largely overcome. By this process a first metallic layer is provided which serves as a good foundation and supporting layer for the deposition of a smooth, uniform and compact intermediate die lectric layer, while the dielectric layer-so deposited upon a hot receiving surface is largely proof against cracking and in its, turn serves as a good supporting layer for the outer metallic layer. Prior to the deposition of the outer metallic layer the support and the unfinished filter thereon should be allowed to return slowly to room temperature. I

In accordance with the invention the vacuum chamber in which the deposition of the layers is carried out is provided with a. filament by means of which the filter supports may be heated by radiant energy to the temperature most conducive to the production of dielectric layers having the H desired properties. The layers may be controlled as to thickness by evaporating tocornpletion from filaments placed in appropriatearrangement ac-'" curately weighed quantities of' the materials to be deposited. Complete evaporation of the materials on the evaporating filaments may be insured by flashing the filaments to a very high temperature. towards the close of the evaporation process. mined quantities of material with a given arrangement of evaporating filaments andfilter supports provides better control of the thickness The evaporation of empirically deter Fig. i is a sectional view on the line iii of Fig. 1 showing one of the evaporating filaments and crucibles used for the evaporation of the dielectric material.

Fig. '1 shows a perspective view of an apparatus adapted to-thepractice of the present invention. A smooth metal plate 20 forming the top of a pump table supports a bell jar 2| sealed to the metal plate by a rubber gasket 22 and within which'theproduction of the filters takes place. The bell jar 2| is evacuated through the conduit 23 by means ofsuitable pumps (not shown) below the plate ZOQ The plate 20 is pierced by four electrodes 30, 3| 32 and 33 within the area covered by the'bell jar and which are insulated from the plate 29 by means of insulating bushings 36. These electrodes serve to provide current to the evaporating filaments and also support the filters an d'the fixture in which they are carried during the process of manufacture. Two additional electrodes 34 and 35'provide current for the heating filament used to heat the filters during the deposition of the dielectric layers. The electrodes 30 and 32 are provided with shoulders at their upper ends on which may be rested either of the filament ring assemblies used in the evaporation of the materials to be deposited, and which support, intheir turn, the filter carrying fixture and the heating system. ,In Fig. 1 is shown supported on the electrodes 30 and 32 the filament ring assembly used for the deposition of the dielectric layers. It comprises a pair of concentrically spaced copper rings and 4| each of which is provided with an inwardly extending'lug 42. These lugs are disposed at theends of a diameter of the rings and are adapted to fit over the upper ends of the electrodes 30and 32 and to rest thereon. i The inner ring ll is split into three equal sectors 43, 44 and 45 of approximately 120 which are spaced from each other by circumferential gaps. The sectors 43, 44 and 45 are assembled into a rigid structure withthering 48 by means of studs 46 which pass through insulating bushings 41. The upper surface of the rings 4!] and 4| is provided with eight pairs of binding posts 50 for connecting evaporating filaments 54 I between thering 40 and the ring 4 I, so that three pieces "of the dielectric to be evaporated. The

sectors '44 and '45-are connected by suitable leads and to the electrodes 3| and 33.

aiascyaar The filamentring'assemblyused in evaporatingthe metal layers 'is similar to that used for the deposition-ofthe dielectriclayers, except that-the ring 4 l= is 1 continuous so that I all eight filamentswillibe energized at 1 once-upon applying a potential betweenthe rings 40'and4 l". Thefilaments employed for the evaporation or the metal consist of tungsten wire wound into the form of ahelix into which the metal to be evaporated is placed.

The 'filter carrying fixture andthe heating-tha ment 'used'" to heat the filters during the -dep'o'si-- tion of the dielectric layer are supported above the filament ring assembly by upright standards Sllwhich rest on the filament ring'assembly. The lower end's'of' the standards'fi'll" are slotted and carry insulating insertswhich fit snugly over the ring 40. The upper portions of-the standards 60 are provided with lengthwise slots 65 so as to makeadjustable the separation'of the filtercarrying fixture supported by the standards fromth'e evaporating filaments" on the" filamentring as-' sembly below. The filter carrying fiXt'urajof which a partial elevation appears in Fig; 2, comprises a circular" plate" "havingfapproXimately the'same diameter as theouter filament ring 40. It is provided with studs'll' extending radially outward from its circumference and adapted to pass'through the slots 65' of the standards B0. Bymeans ofnuts ld thepla'te 10 may be" fastened at anyheight along'the length of the slots 65'. The plate!!! is drilledto provide bearings through which passrotatin'g shafts 81. The shafts 8! are hollow." and accommodate inner shafts 82'from which are suspendedlthe individualfilterholders' 83-. The filter holders 83 consist of metallic rings havinglairecess of diameter equal to the diagon'al'length of the filter supports on which the interference layers are to be deposited; A hanger fifi is'brazedt'o each ring. at diametrically opposite points and fastens to an inner shaft 82. The inner shaftl82 may be inserted through a shaft fll and prevented from falling outxby a thumb nut 86'. The shafts 8| and 82 andthe filter holders 83 may be rotated at a common r'ate'by means of a gear train provided on the upper surface of the plate T01 Each of the shafts. 81 is provided at its upper end. with a gear 90 and the gears are so disposed that each. gear meshes with anadjacent gear. edge ofthe plate Ill is mounted a'pinion 91 on a shaft 92. passing through. the: plate Ill and to which power may-be. applied througha univer-- sal joint 83, an extensible-shaft 94, a second-universal joint 95-and ashaft 96 extending" througha vacuum-tight bushing 91 i in the plate. -to a: power source (not shown) below the: plate 20.

The lower side of the plate l0=isprovided with a number of stand-off'insulatorslflfl. A'heating filament H D: for the heating of the filters during the deposition of the-dielectric'layerisarrayed under the plate 10 in:aplane'subst'antially parallelto-that of the 'filtersinzthe filter. holders 83 by stretching from-theinsulators "3B; The ends ofthe filament. H0 are connected'by suitable-leads to the electrodes-Mend which leadon' the une derside of the plate 20 to the-terminals of' the secondary winding" 01" a suitable variable-transformer (not shown): Theelect-rode?3ll-contacting the outer ring it of thefilament ring assembly through-a lug 62 leads below the plater2ll. to- 'one. terminal of the secondary winding: ofia similar variable: transformer (not shown) ,andithe electrodes-3=l-, 32 andBQ-maybaalternately connected;

Near the D 6. through: a selector switch: to the: other. terminall of that secondary winding.

Fig. 3 an enlarged drawing of: a, microscopic view of 'the surface of. a filter: wherein. th'e.;ma nesium fluoride dielectric layer was. deposited i while the glass support and first silver: layer were at room" temperature; shortcoming of filters producedaccordingitotpre viousmetho'ds and which is overcome" by ourxin ven'tion; A regular network of cracks is= ob served; the length of the cracks averaging: from to 19 microns between intersections; Such filitersshow ma l sedscattering and cloudinessiin'. oblique-light and have a correspondingly reduced: transmission.

In the practice of the invention it has been:

found 'that' the difiiculty of producing=--a= dielec= trio layer'of uniform thiclcnesson all otthefiltei sis bestovercome by evaporating the dielectric from eight filaments arranged in a circlesepa rated fiom the' plane of the filtersby-a distanceequal to" the diameter of this circle. "fiyiusing.

a substantial number of? filaments arranged in:

a circle and byrelating the separation of the itorrrthiscircle as above indicated'; the

n: in thickness of the deposited layers minimized withina single filter'a'n'd' amongthe' The optimum temperature or the glasssupp'ort" and first metallic layer for the reception of 'tl'fc dielectriclayer has been found to'lie between 110 and 146 C.

following example illustrates the -practice o-fzlthe present invention.

A number of squares of pitch-p'olished mirror plateglass sufficient to fill the rotatingiholdersof the fixture of Fig. 1 were carefully cleaned by swabbing with distilled acetone, scrubbihgawith a'calcium carbonate paste followed byscrubbing with a diluted solution of a commercial wettihg agent: and finally by rinsing with hot distilled waters-free acetone.

These filter'supp-orts were thenrnounted; cleanside down, in the rotating filter holders of the filter carrying fixture and introduced into the vacuum chamber with as little delay a's possible. The height of the fixture was-adjusted so that thefilter supports lay in a plane some 7 inchesabove the plane of the evaporating filaments which were disposed in a circle some. 15 inches in diameter. The heating filament used" to heat theunfinished filters during the deposi* tion ofthe'dielectric layer wasdisposed in a plane parallel to the plane of" the filter supports approximately of an-inch above them.

For the deposition of themeta'llic layers the filament ring assembly embodyingan" integral internal-z ring was used so that all evaporating filaments were energized at once;

Each of eight evaporating filaments formed of ahelical-spiral of tungsten wire and making contact with the two" electrical heating rings was loaded-withexactly 59 milligramsof chemically pure silver wire.

The belljarwas then lowered and sealed-onto the: pumping. table and the resulting chamber' was evacuated to a pressure of approximately 51 i0r millimeters of mercury.

l he filter holders were then set intdrotation at aarat'e' of six revolutions per minute-bymeans of the drive shaft passing through the pump table. and the 8 tungsten filaments beari'ngi'th'e' silver to be evaporated were brought up" over'aperiod of one minute to the melting point" of silver. The temperature of the filaments was It illustrates the;

7 then raised until evaporation of the silver was evident and the silver was evaporated to completion over a period of approximately two minutes, the end of the process being observed from the disappearance of dark spots on the filaments.

The vacuum was then broken and the pressure in the vacuum chamber was allowed to rise slowly over a period of approximately three minutes to atmospheric in order to minimize the accumulation of dirt on the filters from the admitted air. The bell jar was then removed and two of the filter supports bearing a silver layer were removed from the fixture and their transmission for the light from a tungsten filament was measured. Transmission values between and 15% indicated satisfactory completion of the evaporation 'of the first silver layer and the two filter supports were returned to the fixture.

For the evaporation of the transparent layer of metallic fluoride the filament ring assembly of Fig. 1, permitting successive evaporation from three groups of filaments, was substituted for the simple filament ring assembly used in evaporating the silver so that the same advantageous use of a large number of filaments properly spaced apart could be retained without generating the excessive heat which would be produced by the simultaneous operation of eight filaments at a temperature high enough to evaporate magnesium fluoride.

In order to produce a batch of filters with second order transmission peaks in the vicinity of 500 millimicrons, each filament was loaded with 100 milligrams of magnesium fluoride. The magnesium fluoride had been previously purified by sintering under a pressure of 10* millimeters of mercury in order to dry and out-gas it. This step is important in order to prevent spattering of the magnesium fluoride during the evaporation process.

After replacement of the filter carrying fixture, the bell jar was again lowered and the chamber was exhausted as before to 5x 10- millimeters of mercury. The filter holders were set in rotation and the heating filament strung above the filter supports from insulators on the underside of the filter carrying fixture was then energized so as to raise the temperature of the filter supports over a period of 25 minutes to a temperature of approximately 125 C. One group of three evaporating filaments was then slowly brought up to white heat over a period of five minutes by energizing one of the sectors of the inner filament ring with respect to the outer ring. When the proper temperature was reached, the magnesium fluoride in the crucibles under these three filaments melted down, whereupon the temperature of these filaments was raised so as to evaporate approximately one-half the magnesium fluoride thereunder over a period of three minutes. The first group of evaporating filaments was then allowed to cool, while the next group was slowly brought up to temperature and the filaments on the other two sectors of the heating rings were successively put through the same cycle. The process was then repeated for all three groups of filaments so as to evaporate to completion all of the magnesium fluoride present. After the evaporation of the magnesium fluoride was complete, the heating filament was deenergized and the unfinished filters were allowed to cool for 25 minutes.

The vacuum was then broken and the pressure slowly raised over a period of five minutes until the bell jar could be lifted and the filter carrying fixture and filament ring assembly removed from the table. The filament ring assembly with an integral inner ring 4! was then replaced on the supporting electrodes 30 and 32 for the deposition of the second silver layer, each of the eight filaments being loaded as before with 59 milligrams of chemically pure silver. The second silver layer was then deposited by the same procedure as the first. After the evaporation of the second silver layer was complete, the pressure within the bell jar was returned to atmospheric over a period of some 25 minutes, after which the filters could be removed from the holders.

The filters were then visually inspected for defects and their spectral transmission curves were taken by means of a grating spectrophotometer to eliminate any exhibiting inadequate transmissions. The filters not rejected were then ce' mented with a properly chosen blocking filter of the absorption type so as to protect the interference layers from the atmosphere and to remove transmission peaks of undesired orders.

The blocking filters were cemented to the interference filter with a self-polymerizing cement according .to the usual techniques used in the cementing of lenses.

The filters so produced did not develop the cloudy appearance and reduced transmission characteristically apparent in filters produced by prior techniques after the cementing operation.

While we have described our invention with reference to one type of apparatus suitable to the practice thereof, it is to be understood that the invention is not limited to the details thereof but that various modifications may be made both in procedure and in theapparatus used within the scope of the appended claims.

We claim:

1. In the manufacture of optical filters having predetermined transmission characteristics and comprising a smooth surfaced transparent support bearing two discrete semi-transparent metallic layers separated by a continuous substantially transparent layer of a metallic fluoride having a uniform optical thickness substantially equal to a small odd number of quarter wave lengths for light of a wave length to be transmit ted by the filter, the method comprising the steps of depositing by evaporation in a vacuum chamber a first semi-transparent metallic layer on said support while said support is at substantially room temperature, heating said support and said first metallic layer in the said vacuum chamber to a temperature substantially within the range of to C., depositing by evaporation in said vacuum chamber upon said first metallic layer while so heated a substantially continuous layer of a metallic fluoride having an optical thickness substantially equal to a small odd number of quarter wave lengths for light of a wave length to be transmitted'by the filter, allowing said support and the layers thereon to return to room temperature in said vacuum chamber, and depositing by evaporation in said vacuum chamber on said layer of metallic fluoride an outer semi-transparent metallic layer while said support and the layers thereon are at substantially room temperature.

2. In the manufacture of optical transmission filters having predetermined transmission char acteristics and comprising a smooth surfaced transparent support bearing two discrete metallic layers each having a uniform transmission for White light not substantially less than 5% nor more than 15% and separated by a continuous a s nal substantially transparent layer of .a metallic fluoride having a'uniform optical thickness substantially equal to a small nddmumber ofgguarter wave lengths forllightmof :aawave length itogibe transmitted by the.filter,..theimethod comprising "layerivvhilelso.heateda substantially transparent continuous layer of -a metallic fluoride by the evaporation in a vacuum chamber of a previously weighed =quantity o'fsaid metallic fluoride suflicient to 'producefa layer thereof "having a uni form optical thickness substantially equal to a small odd number of quarter wave lengths for light of a wave length to be transmitted by the filter, allowing said support and the layers deposited thereon to return gradually to room temperature, and depositing upon said layer of metallic fluoride while at room temperature a second semi-transparent metallic layer by the evaporation in a vacuum chamber of a previously weighed quantity of a metal sufficient to produce a layer having a transmission between substantially 5% and for white light.

3. In the manufacture of optical transmission filters operating by light ray interference and absorption and comprising a clear glass plate bearing two semi-transparent layers of silver separated by an intermediate layer of a metallic fluoride, the method comprising the steps of depositing by evaporation under reduced pressure within a vacuum chamber a semi-transparent substantially continuous layer of silver on a surface of said plate while said plate is at a temperature within the range of substantially to 30 C., thereafter heating said plate and the layer of silver thereon to a temperature within the range of substantially 110 to 140 C. by radiant energy within said vacuum chamber, depositing upon said layer of silver by evaporation within said vacuum chamber and while said plate and layer are so heated a layer of a metallic fluoride having a uniform optical thickness substantially equal to a small odd number of quarter wave lengths for light of a wave length to be transmitted by the filter, gradually lowering the temperature of said plate and the layers thereon in said chamber to substantially the said initial temperature of the plate at which the said layer of silver was deposited, and depositing by evaporation within said vacuum chamber a second semi-transparent substantially continuous layer of silver on said layer of a metallic fluoride.

4. The method of manufacturing optical transmission filters operating by light ray interference and absorption and comprised of a smooth surfaced support bearing two semi-transparent silver layers separated by an intermediate layer of a metallic fluoride and having transmission maxima at pre-selected wave lengths which comprises depositing upon said support while said support is at room temperature a first silver layer by the evaporation in a vacuum of a weighed quantity of silver sufiicient to produce on said support a continuous silver layer of uniform thickness having a transmission for white light not substantially less than 5% and not substantially more than 15%, slowly heating said support and said first silver layer to a temperature withi n the" range of substantially -l10 -to !14Q;C. depositingppon s'aidfirstsilver layer while said support andsaid first silver layer .are at-a temperature within the range of substantially-zllil to 140 C. a continuous transparent .layerof a metallic fluoride bythe evaporation in .a vacuum of e a weighed quantity of said metallic fluoride suflicient -to :produce on said first silver layersa discrete continuous layer of said. metallic fluoride haying a uniform optical thicknesssubstantiallyiequal to .a small 1 odd number of quarter Wave lengths for one-of said pre-selected'iwave lengths =-of maximum transmission, .allowingssaid support andthelayers thereon to return :gradually to room temperature, and depositingion said layerof metallic fluoride a second-continuo-us layer of silver by the evaporation iniayacuum of aweighed quantity of silver suflicient to produceia -continuous -layer of silver having a transmission for white light not substantially less than 5% and not substantially more than 15% 5. In the manufacture, of optical transmission "filters operatingby light "ray interference and absorption and consisting of 'a'p'lurality of discrete semi-transparent metallic layers alternated with discrete transparent layers of a metallic fluoride having an optical thickness substantially equal to a small odd number of quarter wave lengths for light of a wave length to be transmitted by the filter, the process which consists in depositing the semi-transparent metallic layers by evaporation in a vacuum while the surfaces receiving the same are at room temperature, and depositing the metallic fluoride layers by evaporation in a vacuum while the surfaces receiving the same are at a temperature substantially within the range of to C.

6. In the manufacture of optical filters having predetermined transmission characteristics and comprising a smooth surfaced support bearing two discrete semi-transparent metallic layers separated by a continuous layer of a metallic fluoride having a uniform optical thickness substantially equal to a small odd number of quarter wave lengths for light of a wave length to be I transmitted by the filter, the method comprising the steps of depositing by evaporation in a vacuum chamber a first semi-transparent metallic layer on said support while said support is at substantially room temperature, heating said support, and said first metallic layer in the said vacuum chamber to a temperature substantially within the range of 110 to 140 C., depositing by evaporation in said vacuum chamber upon said first metallic layer while so heated a continuous layer of a metallic fluoride having an optical thickness substantially equal to a small odd number of quarter wave lengths for light of a wave length to be transmitted by the filter, allowing said support and the layers thereon to return to r oom temperature in said vacuum chamber, and depositing by evaporation in said vacuum chamber on said layer of metallic fluoride an outer semitransparent metallic layer while said support and the layers thereon are at substantially room temperature.

'7. In the manufacture of optical transmission filters operating by light ray interference and absorption and comprising a plurality of semitransparent metallic layers alternated with layers of a metallic fluoride of index of refraction substantially different from the index of refraction of said metallic layers, the process Which consists in depositing the semi-transparent metallic layers by evaporation in a vacuum while the sur '11 faces receiving the same are at room temperature. and depositing the layers of said metallic fluoride by evaporation in a vacuum while the surfaces receiving the same are at a temperature substantially within the range of 110 to 140 C.

" l 8; In the manufacture of optical filters embodying a transparent support bearing a plurality of semi-transparent metallic layers each of which is spaced from the adjacent metallic layers by layers of a metallic fluoride having an index of refraction substantially difierent from that of the metal, the process of depositing said metallic layers by evaporation in a vacuum while said support is at room temperature, and depositing said spacing layers while said support is at a temperature substantially within the range of 110 to 140 C.

MEYER H. AXLER.

KENNETH F. TRIPP.

REFERENCES CITED The following references are of record in the file of this patent:

OTHER REFERENCES Strong, Procedures in Experimental Physics, published 1938 by Prentice Hall, pages 182, 183. 

